1. Field of the Invention
This invention relates to pretreating oil shale prior to retorting with organic acid which reduces mineral carbonates content, particularly calcite and dolomite, of the oil shale and increases subsequent retorting yield, particularly, the liquid and aromatic fraction recovery from retorting the organic acid pretreated oil shale. The organic acid pretreatment removes water soluble salts prior to retorting which reduces retorting product oil contamination and hazardous wastes.
2. Description of the Prior Art
Oil shale from both Western and Eastern United States contains mineral carbonates which are desirably removed prior to retorting to increase the oil-retort yield. Mineral acids, such as hydrochloric and sulfuric acid solutions, are known to remove the mineral carbonates from oil shale prior to retorting. Pretreatment of oil shale with mineral acids was reported in Environmental Improvements by Oil Shale Leaching, Mohsen Moussavi and T. F. Yen, in Science and Technology of Oil Shale, edited by T. F. Yen, published by Ann Arbor Science Publisher (1976). Such pretreatment can produce a weight loss of up to 15 percent in Western U.S. oil shale, increasing it's permeability about three orders of magnitude. However, a major drawback of the use of mineral acid is the formation of undesirable, hazardous chlorine and sulfur containing by-products as well as adverse economics.
U.S. Pat. No. 4,396,487 discloses treating oil shale by heating in the presence of steam and acetic acid prior to and after attaining retorting temperatures. The oil shale is heated from an ambient temperature to a final temperature of at least 450.degree. C. for retorting. It is preferred to add about half of the steam and acetic acid during heating and prior to the shale reaching about 120.degree. C. and about half of the steam and acetic acid after the shale has reached about 400.degree. C. The '487 patent teaches that the acetic acid acts as a molecular catalyst which enters into a reaction with the kerogen organic material and converts acetic acid into higher boiling distillable organic material forming a different quality product oil. The '487 patent teaches use of acetic acid in amounts of 1.5% to 3% by weight of the oil shale.
U.S. Pat. No. 4,325,787 teaches an apparatus for retorting oil shale including the introduction of an admixture of steam with acetic acid to the particulate raw material prior to introduction into the retort and in the lower portion of the retort where the temperature is about 500.degree. C. U.S. Pat. No. 2,609,331 teaches a method of pretreating oil shale with hydrogen fluoride or hydrochloric acid to react with aluminum, iron or silicon to produce active catalysts for enhancement of oil production.
U.S. Pat. No. 1,703,192 teaches the digestion of oil shale in a heavy mineral oil at about 700.degree. F. in the presence of a basic material, for example, caustic lime, to liberate nitrogen as ammonia and to bind reactive sulfur present. U.S. Pat. No. 4,545,891 teaches the extraction of kerogen from oil shale by dispersing the oil shale in fused alkali metal caustics as a treating agent for releasing kerogen from the oil shale at a treating temperature of 250.degree. to 400.degree. C. U.S. Pat. No. 4,493,762 teaches the extraction of nitrogen from a shale oil product by sulfuric acid treatment.
U.S. Pat. No. 4,243,511 teaches a method for retorting oil shale utilizing superheated water vapor at temperatures from 425.degree. to 510.degree. C., at a superficial gas velocity of at least 10 cm/sec, and a pressure of about 6.9 to 1034 kPa with maintenance of a carbon dioxide partial pressure sufficient to effectively suppress decomposition of alkaline carbonates to obtain an environmentally acceptable retorted shale. U.S. Pat. No. 3,058,904 teaches a method for retorting oil shale wherein solids up-flow combustion retorting is integrated with hot gas eduction in a fluid-downflow retort with maintenance of the eduction gas above about 0.10 atm. reducing heat loss from carbonate decomposition. The '904 patent recognizes that excessive decomposition of carbonates is a limiting factor in many of the shale retorting methods. U.S. Pat. No. 4,241,951 teaches recovery of magnesium values from inorganic carbonates present in oil shale by introducing oxygen to the trailing side of an advancing combustion zone which decomposes kerogen and coverts magnesium carbonates to oxides and hydroxides which may be leached by contacting with an acidic aqueous leaching agent containing a minor amount of a polyelectrolyte to form an enriched solution containing magnesium values.